Conventionally, a combustion apparatus, such as an internal combustion engine, uses a spark plug for igniting an air-fuel mixture through spark discharge. In recent years, in order to meet demand for high output and low fuel consumption, a plasma jet ignition plug has been proposed, since the plasma jet ignition plug provides quick propagation of combustion and can more reliably ignite even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
Generally, the plasma jet ignition plug includes a tubular insulator having an axial bore, a center electrode inserted into the axial bore in such a manner that a front end surface thereof is retracted from a front end surface of the insulator, a metallic shell disposed externally of the outer circumference of the insulator, and an annular ground electrode joined to a front end portion of the metallic shell. Also, the plasma jet ignition plug has a space (cavity) defined by the front end surface of the center electrode and a wall surface of the axial bore. The cavity communicates with an ambient atmosphere via a through hole formed in the ground electrode.
Additionally, such plasma jet ignition plug ignites an air-fuel mixture as follows. First, voltage is applied between the center electrode and the ground electrode, thereby generating spark discharge therebetween and thus causing dielectric breakdown therebetween. In this condition, high-energy current is applied between the center electrode and the ground electrode for effecting transition of a discharge state, thereby generating plasma within the cavity. The generated plasma is discharged through an opening of the cavity, thereby igniting the air-fuel mixture. For example, see Japanese Patent Application Laid-Open (kokai) No. 2007-287666 (“Patent Document 1”).